The effect of evaporation on size and shape evolution of faceted gold nanoparticles on sapphire O. Malyi, E. Rabkin ⇑ Department of Materials Engineering, Technion-Israel Institute of Technology, 32000 Haifa, Israel Received 9 June 2011; received in revised form 2 September 2011; accepted 25 September 2011 Available online 29 October 2011 Abstract We studied the size and shape evolution of about 180 faceted gold nanoparticles attached to a sapphire substrate during annealing at 950 °C in air. We employed the scanning force microscopy and interrupted annealing techniques to track the changes in size and shape of individual nanoparticles. The height of all single-crystalline nanoparticles was constant up to the longest cumulative annealing time of 65 h. The lateral dimensions of 20% of all nanoparticles shrunk during anneals, while all three dimensions of the remaining 80% of nanoparticles remained constant. Only the nanoparticles with the height below the average (for all particles) were laterally shrinking. We formulated a kinetic model relating the lateral shrinkage of the nanoparticles to the evaporation of Au atoms adsorbed on sapphire. We also assumed that the process controlling particles shrinkage is the slow self-diffusion of Au atoms along the lateral facets of the nanoparticles toward the substrate. The model predicted a power law dependence of the shrinkage rate on the particle height, with the exponent n = 3. The corresponding exponent determined from the experimental data was n = 2.9 ± 0.3, in excellent agreement with the theory. The low value of the effective self-diffusion coefficient along the lateral facets determined with the aid of our model (3.2 ± 0.2  10 17 m 2 s 1 ) was attributed to the difficulties of step nucleation on atomically flat facets. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Anisotropy; Surface energy anisotropy; Diffusion; Theory and modeling; Capillary phenomena 1. Introduction Arrays of noble metal nanoparticles supported by inert ceramic substrate find important applications in catalysis [1], in growth of semiconductor nanowires [2], and in pho- tonic structures [3]. In one of the “top-down” approaches, such arrays are produced employing the dewetting of thin metal films deposited on ceramic substrate. Because of the poor wettability of ceramic substrates by both liquid and solid metals, a thin metal film breaks down into iso- lated particles upon annealing, the process being driven by the decrease of the total surface and interface energy of the system. Once the isolated solid metal particles are formed, their further size, shape and spacing evolution depends on annealing temperature, time and atmosphere. The coarsening of the particles, evaporation of the metal, and diffusion of metal atoms into the substrate were iden- tified as the most important factors determining the size and spacing evolution of the particles during annealing in the solid state [4,5]. The coarsening is a matter-conserving mechanism of the size increase of large particles at the expense of their small counterparts. The process is driven by the gradient of chemical potential caused by the depen- dence of the chemical potential of surface atoms on particle size (Gibbs–Thomson effect). The diffusion of metal atoms through the vapor phase, the bulk diffusion of metal atoms in the substrate, and the diffusion of absorbed metal atoms along the substrate surface were proposed as possible mechanisms of coarsening, the latter being consid- ered as the most probable mechanism in the majority of studied systems [4]. The evaporation of metal leads to the loss of the total amount of metal on the substrate and may play a significant role during annealing in dynamic 1359-6454/$36.00 Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2011.09.045 ⇑ Corresponding author. Tel.: +972 4 829 4579; fax: +972 4 829 5677. E-mail address: erabkin@tx.technion.ac.il (E. Rabkin). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 60 (2012) 261–268